1. Technical Field
The present invention relates to the Asymmetric Digital Subscriber Line (ADSL) connections established between an access node of service provider network such as the Internet network and a user workstation, and relates in particular to a system and method for dynamically inverting an ADSL system.
2. Description of the Related Art
Modems are used to enable two computers to communicate via the Public Switched Telephone Network (PSTN). The latter carries only analog signals and modems are used to translate digital data from a computer into a series of high-frequency signals that can be transported over phone lines. When such analog signals arrive at the destination, they are demodulated into digital data for the receiving computer.
Digital Subscriber Line (DSL) modems provide a digital subscriber line within the extant PSTN network. A DSL modem transmits duplex data at higher speed than conventional modems. Such DSL modems use a twisted pair having a bandwidth from 0 to approximately 80 kHz, which precludes the simultaneous use of analog telephone service in most cases.
Asymmetric Digital Subscriber Line (ADSL), a new modem technology, belongs to the DSL family and converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. ADSL transmits more than 6 Mbps to a subscriber or user premises, and as much as 640 kbps in the reverse direction. Such rates expand existing access capacity by a factor of 50 or more without requiring new cabling. ADSL can transform the existing public information network from one limited to voice, text and low resolution graphics, to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone's home this century.
The ADSL system will play an important role over the next ten or more years as telephone companies enter new markets for delivering information in video and multimedia formats. New broadband cabling will take decades to reach all prospective subscribers. The success of these new services will depend upon reaching as many subscribers as possible during the first few years. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homes and small businesses, ADSL will make these markets viable, and profitable, for telephone companies and application suppliers alike.
Many applications foreseen for ADSL involve digitally compressed video. As a real time signal, digital video cannot use link or network level error control procedures commonly found in data communications systems. ADSL modems therefore incorporate forward error correction that dramatically reduces errors caused by impulse noise. Error correction on a symbol-by-symbol basis also reduces errors caused by continuous noise coupled into a line.
In an ADSL system, there is an ADSL modem on each end of a twisted-pair telephone line, creating three information channels: a high-speed downstream channel; a medium-speed duplex channel, depending on the implementation of the ADSL architecture; and a POTS (Plain Old Telephone Service) or an integrated services digital network (ISDN) channel. The POTS/ISDN channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS/ISDN, even if ADSL fails. The high-speed channel ranges from 1.5 to 6.1 Mbps, while duplex rates range from 16 to 640 kbps.
The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex channel. ADSL modems will accommodate ATM transport with variable rates and compensation for ATM overhead, as well as IP protocols. Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases.
Each ADSL channel can be submultiplexed into multiple lower rate channels. To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways, Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low-speed channels. The upstream path is also multiplexed into corresponding low-speed channels. Echo Cancellation assigns the upstream band to overlap the downstream, and separates the two by means of local echo cancellation, a technique well known in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band.
The asymetric nature of ADSL, however, does not enable an ASDL system to utilize the overall bandwidth for some applications wherein the user workstation acts as a server for the transmission of large files, a video conference, or a data distribution. Conventional ADSL systems are thus only suitable for applications requiring high-speed transmission in a single direction.